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24.
CORRELATING FLOODS TO OVERLAND PATH RISK
• 98% of flooded properties are beyond river flood
vulnerable areas (part of regulated valley areas) and
are on table land, impacted by overland surface flow.
• Statistically, properties with reported basement
flooding are closer to the overland surface flow path.
• On average, for the closest 30% of properties, flooded
properties are located at 58% of the distance to the
overland path of all other properties (control group of
500k+ addresses).
• For the next closest 30% of properties, flooded
properties are at 74% of the distance of the control
group, and remaining flooded properties are at 93%
distance (less correlation farther away).

32.
EXAMPLE OVERLAND RISK MAPPING
• All Toronto addresses have been classified by proximity
to overland flow, as multiples of 100 year flow spread.
31% are within 4 times the 100 year spread.

33.
BACK-UP AND OVERLAND PERIL CONSIDERATIONS
• WHAT IS RELATED?: Overland flood risks and basement back-up
risks are shown to be correlated. Traditionally, back-up is associated
with the ‘minor’ sewer system and not the ‘major’ overland system.
• HOW ARE RISKS RELATED?: Sewer back-up can occur in spatially
random locations based on local factors / deficiencies, but is shown
to be more often clustered for severe storms, reflecting
neighbourhood scale factors including overland drainage system.
• WHERE ARE THE RISKS?: 98% of basement flooding is on table land.
Riverine food risks are low where flood hazard regulations are
enforced, such that only 2% of Toronto severe weather flooding is
within ‘flood vulnerable’ areas along valleys.
• CAUSE?: Overland risk factors are seldom mapped or managed.
Pre-1980’s drainage design did not preserve overland system. Infill /
intensification can increase overland flows, adversely affect
overland drainage path (filled in / blocked), & stress minor system
with overland inflows (into homes, ponding/inflows over MHs).